1. Field of the Invention
The present invention is directed to a linear interpolator for use in a digital convergence system for providing vertical as well as horizontal smoothing of convergence correction signals.
2. Description of the Prior Art
A display on a cathode ray tube (CRT) comprises a plurality of scan lines disposed thereon. The scan lines are traced along the inner surface of the CRT by electron beams which are generated from within the CRT, the intensity of the electron beams being controlled as the beams are scanned across the inner surface of the CRT, typically from left to right, top to bottom. Since the electron beams are often misconverged during the scan, convergence correction is required in order to ensure that the electron beams, usually three in number, converge together at the proper points along the inner surface of the CRT during their scan thereacross. A digital convergence system provides this convergence correction by synthesizing a convergence waveform, the waveform energizing convergence coils in the neck of the CRT for deflecting one of the electron beams and thereby providing the necessary convergence. A similar convergence waveform energizes said coils for deflecting the other electron beams. The convergence waveform is synthesized in the following manner.
Referring to FIG. 1, the CRT screen is divided into an N by N matrix of blocks 10, each of the blocks having a digital value corresponding thereto. For example, a 16 by 16 matrix could be utilized. The electron beams may scan, for example, from left to right, top to bottom across the blocks along the inner surface of the CRT. When the electron beams begin tracing a first scan line. in the first row 10A of the matrix, the first block 10A1 is addressed. A digital value corresponding to said first block 10A1, is generated, the digital value being used to synthesize the convergence waveform. When the electron beams address the next horizontal, adjacent block 10A2 along the first scan line of said first row, another digital value corresponding to said horizontal, adjacent block 10A2, is generated. As the electron beams continue to scan in the remainder of said first scan line of said first row 10A, across other blocks along the inner surface of the CRT, other digital values are generated. Each of the digital values represent a step-function, and each of the digital values are concatenated together. When each of the sequentially generated digital values are concatenated, a voltage waveform is developed associated with said first scan line of said first row 10A. The voltage waveform has a plurality of horizontal discontinuities associated therewith, a horizontal discontinuity being an abrupt increase or decrease in voltage, occurring between each adjacent concatenated step function of the waveform.
Conventional filtering techniques are used to convert each horizontal discontinuity in the voltage waveform. into a smooth transition thereby producing a continuous convergence waveform associated with said first scan line of said first row. The continuous convergence waveform energizes the convergence deflection coils in the neck of the CRT for deflecting one of the three electron beams. The other two of the three electron beams are deflected in a similar manner as hereinbefore described thereby converging the three electron beams. The three converged, deflected electron beams produce said first scan line of said first row. One technique for converting each horizontal discontinuity in the voltage waveform into a smooth transition is present in IBM Technical Disclosure Bulletin, Vol. 21, No. 1, June 1978, entitled "CONVERGENCE CORRECTION FOR CRT DISPLAYS," by M. Brandon.
The electron beams retrace to their original position and begin to scan along the second scan line of the first row 10A beginning with the first block 10A1. Another continuous convergence waveform is developed associated with the second scan line in the same manner as hereinbefore described with respect to the first scan line of said first row 10A.
The first and second scan lines are disposed within the same row 10A. Therefore, the same digital values will be generated for each scan line; and the same continuous convergence waveform will be developed for each scan line. As a result, the first and second scan lines will appear to be approximately parallel to one another.
Other continuous convergence waveforms are developed associated with the intermediate scan lines of the first row 10A. When the electron beams begin to trace the last scan line of the first row 10A, beginning with block 10A1, still another convergence waveform is developed in the same manner as hereinbefore described. However, when the electron beams begin to trace the first scan line of the second row 10B, the first block 10B1 of the second row 10B is addressed. The digital values in the blocks of the second row 10B may be different than each of the respective digital values in the blocks of the first row 10A. Since the digital values in the blocks of the second row 10B are different, a different convergence waveform is developed associated with the scan lines in the second row 10B relative to the scan lines in the first row 10A. Consequently, the scan lines of the second row 10B will not appear to be parallel to the scan lines of the first row 10A. The different convergence waveform associated with the scan lines of the second row 10B relative to the first row 10A, and the resultant non-parallel relationship therebetween, results in a visual discontinuity, that is, an abrupt transition between the first row and the second row, and in particular, between the last scan line of the first row 10A and the first scan line of the second row 10B. This abrupt transition is termed a vertical discontinuity.
Reference is directed to FIG. 2 of the drawings wherein a distorted raster field is illustrated. A raster field, by definition, is a plurality of scan lines traced by the electron beams on the inner surface of the CRT, the scan lines being traced within a portion of the matrix of blocks 10 shown in FIG. 1. Since the digital values associated with the blocks of the first row are different than the digital values associated with the blocks of the second row, a vertical discontinuity 12 appears between the last scan line of the first row 10A and the first scan line of the second row 10B. Since the digital values associated with the blocks in the second row 10B are different than the digital values associated with the blocks in a third row 10C, respectively, another vertical discontinuity 14 appears between the last scan line of the second row 10B and the first scan line of the third row 10C. In fact, in FIG. 2, vertical discontinuities appear between each of the rows due to the different respective digital values associated with each of the blocks in each of the rows. The vertical discontinuities (e.g., 12 and 14 in FIG. 2) cannot be filtered using the conventional filtering techniques because these filtering techniques are used to ease the transition between two adjacent points on an output waveform, such as the convergence waveform. Since the digital convergence system of the prior art did not take into account the different digital values associated with each of the blocks of one row relative to the respective blocks of the next adjacent row, the prior art convergence system did not ease the abrupt transition between groups of scan lines disposed in two adjacent rows of the matrix. Therefore, the vertical discontinuities appeared in the raster field, disposed between adjacent rows of the matrix.
These vertical discontinuities do not present a severe problem with respect to a character oriented display as long as the location of each of the characters in the character oriented display is limited to the center of a matrix block. However, for a graphics display, vertical discontinuities can present a severe problem. For example, when generating a pie-chart on a CRT display, if one section of the pie should be filled with a red fill pattern, a distorted raster field, such as that which is illustrated in FIG. 2, having a plurality of vertical discontinuities present therein, may result in gaps in said one section, such as discontinuity 12 of FIG. 2, or in bright red areas in said one section, such as discontinuity 14 of FIG. 2.